1. Field of the Invention
This invention relates to compositions which cure under anaerobic conditions.
2. Description of the Art Practices.
It has long been known that such compositions which will cure under anaerobic conditions may be prepared. Such compositions may be used for locking bolts onto nuts to guard against vibration loosening of the nut. Recently, great interest has been placed in anaerobic curing compositions for use as sealants. That is, the sealant area of activity utilizing anaerobic adhesives is a replacement for conventional rubber plastic gaskets in applications such as pumps and internal combustion engines.
The anaerobic adhesive compositions of the prior art function as follows. First, a polymerizable component is employed which will upon proper activation polymerize. Materials such as cumene hydroxide are often utilized to initiate the reaction in the polymerizable component which may be conveniently a methacrylate or acrylate compond. Ordinarily, such compositions would be inherently unstable due to the presence of the polymerizable bond and the free radical initiator within the composition. To overcome this problem, the art has suggested using materials which are stabilizing inhibitors. The stabilizing inhibitors, generally quinone compounds, act as a free radical sink thereby picking up free radicals generated by the initiator thus stabilizing the composition. A complementary method of stabilizing anaerobic curing compositions is to allow air to permeate the container in which the composition is stored. The oxygen molecules in the air have the capability to act as a free radical sink in addition to the stabilizing inhibitor. There is, however, a penalty for allowing air to permeate the container of the anaerobic composition which is that such containers are considerably more expensive than lined metal pails. In most cases, plastics having a high degree of porosity are employed to obtain benefit of oxygen permeation.
There is also a difficulty in utilizing the stabilizing inhibitors such as quinones in an anaerobic composition. This difficulty stems from the fact that the stabilizing inhibitor does not know when it is supposed to cease its inhibiting function. By definition, the oxygen as a free radical sink is excluded when the adhesive function or the sealant is formed. However, the stabilizing inhibitor being a portion of the composition is always present and to the extent it has not been depleted by the free radials will still serve to inhibit the composition when the cure is attempted. Thus, a slight overuse of the stabilizing inhibitor can prevent the composition from curing. Moreover, the anaerobic compositions tend to be stored for varying periods of time by customers. Thus, if the initial level of the stabilizing inhibitor is set too high or too low, the anaerobic cure becomes haphazard. Thus, an ideal anaerobic curing system would avoid the use of stabilizing inhibitors if at all possible.
As noted previously the anaerobic compositions most often include a chemical compound capable of generating free radicals (it should also be noted that it is possible through the use of various forms of radiation to generate free radicals without the use of chemical compounds). The chemical compounds capable of forming the free radical are not only themselves inherently unstable but also will begin to liberate the free radical thereby promoting polymerization almost immediately after addition to the composition. Some free radical generators are not practically usable due to their inherent ability to gel the anaerobic composition even in the absence of anaerobic conditions.
What is most often used in the industry is a material such as cumene hydroperoxide which is not the strongest free radical initiator available. To step up the ability of the free radical initiator in curing the anaerobic composition, and thereby overcoming any remaining stabilizing inhibitor, accelerators are utilized. Accelerators are materials which have the function of a catalyst wherein the free radical initiator is caused to liberate the free radicals on a rapid basis. Such materials known in the art include sulfimides and tertiary amine compounds. In particular, combinations of the sulfimides and tertiary amine compounds appear to cause a synergy when present with a material such as cumene hydroperoxide under anaerobic conditions. Thus, the sulfimides and tertiary amines do not appear to inherently destabilize the composition in the absence of anaerobic conditions.
Of the various attempts in the art to find suitable components for anaerobic compositions is U.S. Pat. No. 4,038,475 issued to Frauenglass et al. on July 26, 1977. This patent describes the use of metal chelating agents to assist in stabilizing the anaerobic adhesive composition. U.S. Pat. No. 3,041,322 issued to Krieble on June 26, 1962 discloses various polymerizable compounds useful in anaerobic compositions.
It is also recommended that the reader review U.S. Pat. No. 3,218,305 issued Nov. 16, 1965 to Krieble as well as U.S. Pat. No. 2,895,950 issued July 21, 1959 also to Krieble.
U.S. Pat. No. 3,043,820 issued July 10, 1962 also contains teachings of Krieble relating to polymers for use in anaerobic compositions. It is also suggested that the patent to Burnett issued Feb. 10, 1953 as U.S. Pat. No. 2,628,178 be reviewed. Further references include U.S. Pat. No. 4,180,640 issued to Melody et al. on Dec. 25, 1979; U.S. Pat. No. 4,245,077 issued to DeMarco Jan. 13, 1981; and U.S. Pat. No. 3,987,234 issued to Gruber et al. on Oct. 19, 1976.
The foregoing patents to the extent applicable are herein incorporated by reference and made a part of the disclosure of this patent. Throughout the specification and claims, percentages and ratios are given by weight and temperatures are in degrees Celsius unless otherwise indicated.